1. Field of the Invention
The present invention pertains to a method and apparatus for providing a reliable end-tidal carbon dioxide (CO2), end-tidal oxygen (O2), or other gas estimation.
2. Description of the Related Art
Respiratory gas monitoring systems typically comprise gas sensing, measurement, processing, communication, and display functions. Such systems are considered to be either diverting (i.e., sidestream) or non-diverting (i.e., mainstream). A diverting gas measurement system transports a portion of the sampled gases from the sampling site, which is typically a breathing circuit or the patient's airway, through a sampling tube, to the gas sensor where the constituents of the gas are measured. A non-diverting gas measurement system does not transport gas away from the breathing circuit or airway, but measures the gas constituents passing through the breathing circuit.
Conventional non-diverting gas measurement systems include gas sensing, measurement and signal processing required to convert the detected or measured signal (e.g., voltage) into a value that may be used by the host system. The gas measurement system communicates with the sample cell placed at the breathing circuit and comprises the components required to output a signal corresponding to a property of the gas to be measured. Placement of the sample cell directly at the breathing circuit results in a “crisp” waveform that reflects in real-time the partial pressure of the measured gas, such as carbon dioxide or oxygen, within the airway. The sample cell, which is also referred to as a cuvette or airway adapter, is located in the respiratory gas stream, obviating the need for gas sampling and scavenging as required in a diverting gas measurement system.
Conventional diverting gas measurement systems utilize a relatively long sampling plastic tube connected to an adapter in the breathing circuit (such as a T-piece at the endotracheal tube or mask connector) or a nasal catheter. The sample gas is continuously aspirated from the breathing circuit or the sample site through the sampling tube and into the sample cell within the monitor at sample flow rates ranging from 50 to 250 ml/min. The location of the sampling port in the breathing varies and may range anywhere from an elbow connected to an endotracheal tube to the wye connector.
Both diverting and non-diverting gas measurement systems include sensors that measure the concentration and/or partial pressure of at least one of the gas components in the sampled gas passing through the sample cell. Two of the most commonly measured gases of clinical importance are carbon dioxide and oxygen. Both diverting and non-diverting gas measurement systems utilize sensors to measure the constituent gases such as carbon dioxide and oxygen.
To measure these gases, electro-optical assemblies are often employed. In the case of a carbon dioxide sensor and a number of other gas sensors, these assemblies includes a source that emits infrared radiation having an absorption band for carbon dioxide. The infrared radiation is usually transmitted along a path that is normal to the flow path of the gas stream being analyzed. Photodetectors are arranged to receive and measure the transmitted radiation that has passed through the gas in the gas stream. Carbon dioxide within the sample gas absorbs this radiation at some wavelengths and passes other wavelengths. The transmitted radiation is converted to signals from which a processor calculates the partial pressure of carbon dioxide. In the case of an oxygen sensor, electrochemical or fluorescence based technologies are often employed.
Carbon dioxide and oxygen are expressed either as a gas fraction (FCO2 and FO2) or partial pressure (PCO2 and PO2). Capnography and oxygraphy, when used without qualification, refers to time-based capnography and oxygraphy. In addition to capnometry, capnography includes a plot of the instantaneous carbon dioxide concentration over the course of a respiratory cycle. From this plot, the cyclic changes can be visualized.
In a “textbook” capnogram 2, an example of which is shown in FIG. 1, the capnogram comprises two segments: an “expiratory” segment 4, and an “inspiratory” segment 6. The expiratory segment consists of a varying upslope 5a that levels to a constant or slight upslope 5b. The inspiratory segment consists of a sharp downslope 7a that settles to a plateau of negligible inspired carbon dioxide 7b. However, other than the end-tidal partial pressure of carbon dioxide, which has been generally understood as the partial pressure of carbon dioxide at the end of expiration, only breathing frequency and a measure of inspiratory carbon dioxide levels are clinically reported. This is the case because only the transition between the expiratory and inspiratory segments can usually be well delineated from a capnogram.
Even then, only if there is substantially no rebreathing, does this transition correspond to the time of the actual beginning of inspiration as delineated by the flow waveform. The transition between inspiration and expiration cannot be readily discerned because of the presence of anatomic dead space that fills with inspiratory gas at the end of expiration. Although the oxygram is not in as widespread clinical use as capnograph, the same issues discussed above apply to the oxygram with the understanding that the oxygram can be considered an inverted version of the capnogram.
If flow is measured in addition to carbon dioxide, the volumetric capnogram can be determined. Similarly if flow is measured in addition to oxygen, the volumetric oxygram can be determined. FIG. 2 illustrates the three phases of a volumetric capnogram. Phase I comprises the carbon dioxide free volume, while phase II comprises the transitional region characterized by a rapidly increasing carbon dioxide concentration resulting from progressive emptying of the alveoli. Phases II and III together are the carbon dioxide containing part of the breath, the effective tidal volume, VTeff. Phase III, the alveolar plateau, typically, has a positive slope indicating a rising PCO2. Using these three phases of the volumetric capnogram, physiologically relevant measures, such as the volumes of each phase, the slopes of phase II and III, and carbon dioxide elimination, as well as deadspace tidal volume and ratios of anatomic and physiologic deadspace can be determined.
One of the objectives when setting the level of mechanical ventilation for a patient is to reach and maintain a desired concentration of arterial carbon dioxide concentration (PaCO2). Because real-time access to PaCO2 measurements is not easy, estimates from a capnogram are used to obtain a surrogate measure. Because of pulmonary shunting, i.e., a portion of the right heart cardiac output reaches the left atrium without having participated in gas exchange, the closest surrogate of PaCO2 that can be obtained from the capnogram is alveolar CO2 concentration (PACO2).
The end-tidal partial pressure of CO2 (PetCO2), usually referred to as the end-tidal carbon dioxide, is used clinically, for example, to assess a patient ventilatory status and, as noted above, has been used by some as a surrogate for PaCO2. Similarly, the end-tidal partial pressure of O2 (PetO2), which may be referred to as the end-tidal oxygen, is also used.
The medical literature is replete with conflicting articles regarding the relationship between PetCO2 and PaCO2, as well as the relationship between changes in PetCO2 and changes in PaCO2. On one hand, Nangia et al. notes that “ETCO2 correlates closely with PaCO2 in most clinical situations in neonates”. Similarly, Wu et. al. notes that “we recommend using mainstream capnography to monitor PetCO2 instead of measuring PaCO2 in the NICU.” On the other hand, Russell et al. studied ventilated adults and noted “trends in P(a-et)CO2 magnitude are not reliable, and concordant direction changes in PetCO2 and PaCO2 are not assured.”
Researchers have considered maneuvers to improve ‘prediction’ of PaCO2. Tavernier et al. studied whether prolonged expiratory maneuvers in patients undergoing thoracoabdominal oesophagectomy improved the prediction of PaCO2 from PetCO2 and concluded that these maneuvers did not improve estimation. A commonly held belief among critical care physicians is that end-tidal CO2 cannot be used as a surrogate for either arterial PCO2 or changes in arterial PCO2. To complicate matters further Chan et al. noted that “mainstream PetCO2 provided a more accurate estimation of PaCO2 than side-stream measurement.”
If end-tidal PCO2 could be reliably used as a surrogate for arterial CO2, arterial blood sampling could be reduced, applications that currently use intermittent blood sampling would become more clinically acceptable, and applications, such as closed loop control of ventilation (particularly non-invasive ventilation), would be more viable. Therefore, techniques for reliability and/or indicating the reliability of end-tidal PCO2 estimations are desired.